Many technologies have been designed to allow motorists to carry out the inflation of the tires after running, with no distance constraint.
Since the tires are then at a temperature higher than the standard inflation temperature, if the standard pressure value is used directly, under-inflation is then obtained, the magnitude of which depends on the difference between the real inflation temperature and recommended standard temperature.
For example, if the tires of a vehicle are reinflated in a heated state, after running, on an indicated pressure base of 2.0 bar, when the tires have cooled down, it is probable that the user will obtain a significant pressure difference of the order of 130 to 180 mbar between the front tires and the rear tires. Specifically, on most current motor vehicles, the temperature of the front tires rises more than that of the rear tires. Typically, for a difference of approximately 15° C., the pressure difference is approximately 150 mbar. If inflation is carried out when hot with tires having excess pressure due to the heat, and without taking account of this excess pressure, the result will be under-inflation. Specifically, when the tires cool, the excess pressure disappears. The tires are then under-inflated by a value depending on the magnitude of the excess pressure induced by the heating and present at the time of inflation.
Such a real pressure difference may have an impact on the driving of the vehicle, its road behaviour, its road holding, etc.
According to another typically-known case, the temperature difference between a tires situated in the shade and a tires situated in the sun may reach 15° C. to 20° C. and even more. This causes an induced pressure difference that may reach or even exceed 200 mbar, for example between the tires on the driver's side, sheltered from the sun, and the tires on the passenger side, exposed to the sun. If reinflation takes place in such a context, the real pressure of the tires when the temperatures have returned to a balanced state could be 200 mbar less than the value indicated during inflation when hot and therefore not only would the tires be under-inflated relative to the recommended pressure but also out of balance relative to the opposite side.
Here again, such a real pressure difference may have an impact on the driving of the vehicle, its road behaviour, its road holding, etc.
Therefore, to alleviate this situation, various means and methods are used to compensate for the inflation pressure value.
Document FR2885321 describes a solution consisting in providing means for analysing the pressure and temperature comparing the measured value of the tire's inflation pressure with a recommended pressure. The real inflation pressure is corrected according to the tire temperature indicated by a device for detecting tire temperature. The reference temperature value used is established according to the temperatures measured during the latest start-ups of the said vehicle.
Document FR2667270 deals with the context specific to the inflation of the tires at the factory, in order to compensate for the temperature variations during the inflation process. This objective is achieved by incorporating a means for automatically detecting the temperature of the compressed air used in the inflation of the tires and the ambient temperature of the factory, and in adjusting the predetermined inflation pressure in order to take account of these variables. In addition, the inflation device presented incorporates a means for detecting the real inflation pressure of the wheel/tire assembly during the inflation process in order to have the assurance that the temperature-compensated inflation pressure is really obtained. This device, involving the use of two means for measuring the temperature (one for the ambient temperature and the other for the temperature of the inflation air), is technically difficult to apply on inflation sites intended for the consumer, because such sites have to be low-cost in order to be able to be present in large numbers over the whole road network.
Document GB2388197 describes a device in which the vehicle is fitted on the one hand with temperature sensors making it possible to determine the tire temperature and on the other hand with pressure sensors. Therefore, the inflation may be carried out taking account of the real temperature of the tires.
Document U.S. Pat. No. 5,140,851 describes a device making it possible to compensate for errors in measuring the temperature when the tire air temperature and volume are determined. A correction means based on correction parameters makes it possible to obtain corrected values. Furthermore, the device makes it possible to reveal a possible reduction in the tire air volume.
Document U.S. Pat. No. 6,711,955 describes a device making it possible to inflate the tires in the hot state, in order to prevent having to wait for the latter to cool down before carrying out the inflation. This device involves two temperature sensors in order to detect the temperature of the tire and of the ambient air. A processor makes it possible to compare the temperature and pressure values with a recommended value. This device is particularly suitable for the inflation of aircraft tires.
Document US2005162263 describes a method making it possible to inflate tires while taking account of two distinct temperature values in order to compensate for the effect of this temperature on the pressure. The temperature measurements are taken at the rim and at the brakes of the vehicle.
Document US 2006276990 describes an inflation device provided with a controller storing a target pressure value to which the tire should be inflated when it is at a standard temperature. Data relating to the real pressure and temperature are obtained and are used to determine a refill pressure compensated for according to the temperature.
The document FR 2.829.423 described a system for the detection of a pressure decrease in a tire which contains a probe of pressure of air, a temperature sensor of the tire and a unit of evaluation. The unit of evaluation converts the values of pressure of air and the values of temperature measured in values of pressure of air compensated in temperature, determines the modification in the time of the values of pressure of air compensated and generates a signal of warning if the modification in time exceeds a threshold value. The unit of evaluation decreases the threshold value of modification of pressure of air when the difference between the value of pressure of air measured and compensated in temperature compared to a value of reference of pressure of air predetermined and compensated in temperature increases.
In all these documents, it is accepted that the inflation of the tires in the hot state has serious disadvantages that it is preferable to avoid. All these devices and methods make reference to means making it possible to carry out an inflation at ambient temperature while making a compensation based on the one hand on pressure and temperature measurements and on the other hand on compensation computations. Many sources of errors and/or inaccuracies are present both in the measurements taken and in the computations made for the compensation.
Moreover all these documents propose systems which inform the driver of the vehicle when a problem of inflation exists in an unquestionable way to limit the number of transmitted alarms. In spite of that the messages transmitted to the driver are often not understood and call even into question the confidence of these drivers towards these systems.